The invention relates to the production of tube from thermoplastic material, in particular from polyolefin plastic material, such as polyethylene. The invention also relates to the production of plastic tube in which the thermoplastic material is biaxially oriented, which process is known as the biaxial stretching process. The invention also relates to improvements to the process for the production of extruded tube from thermoplastic material, which process may form part of the production of biaxially oriented plastic tube. The invention additionally relates to the production of an improved joint between tubes made from biaxially oriented thermoplastic material.
The present invention relates in particular to the production of a tube from biaxially oriented thermoplastic material with an integrally formed socket at an end, so that tubes of this nature can be coupled to one another via socket joints, in order in this way to form a pipe, for example for transporting water, gas, etc.
WO 95/25626 has disclosed a method for the production of biaxially oriented plastic tube, also known as a stretched tube. In this method, the stretched tube is of uniform cross section, i.e. has a uniform wall thickness and diameter, over its entire length, and is also uniformly stretched in the axial and tangential (circumferential) direction of the tube over its entire length. A method for providing a tube which has been produced in this way with a socket at one of its ends is known from WO 97/33739.
Another method for producing tube from biaxially oriented plastics material is known from GB 1 589 052. This method is based on a tube made from thermoplastic material which has not been subjected to biaxial orientation, which tube has a tube body with, at one end, an end part with a greater wall thickness than the tube body. The tube is placed in a die and is expanded by an internal pressure so that the plastics material of the tube is biaxially oriented. In the process, the end part is deformed to form a socket.
WO 98/13190 has described yet another method for the production of a tube with an integral socket from biaxially oriented thermoplastic material.
Despite all the developments in the field of the production of tubes from biaxially oriented thermoplastic material, and in particular in the field of forming a socket on a tube of this nature, load tests still show that the socket of a tube of this nature forms the critical part of the tube. This is because the tube has been found to rupture earlier at the socket than in the tube body, and therefore the socket constitutes an undesirable limitation on the mechanical strenght of the tube.
The object of the present invention is to propose measures which make it possible to produce a tube of the above type with an integral socket at one or both ends. The invention also provides measures for improving the spigot of the tube, which is to be fitted into a socket.
For this purpose, the invention, according to a first aspect, provides a method wherein-by periodical variation of the ratio of the preform advancement speed, on the one hand, and the output of the extruder, on the other hand, between a plurality of different values-the wall thickness of the preform is periodically changed.
When the method according to the first aspect of the invention is carried out, an axial preform part with a different wall thickness from the preceding part of the preform is periodically formed in the section between the extruder die and the tube speed-control means, in practice in particular immediately downstream of the extruder die.
Surprisingly, in practice it has proven possible to control the process of biaxial stretching of the preform successfully despite the variation in the wall thickness of the preform which is to be forced over the mandrel. In particular, it has proven possible for a preform part with a greater wall thickness to be forced over the mandrel without this having undesirable effects on that part of the preform which has a smaller wall thickness and is located between the said thick preform part and the drawing device.
The method according to the first aspect of the invention enables a stretched tube of biaxially oriented thermoplastic material to be produced in a continuous process with axial tube parts which have varying wall thicknesses.
In practice, it has proven expedient for the maximum wall thickness of the preform to be 5-15% greater than the smallest wall thickness of the preform, as seen at a location immediately downstream of the extruder die. It will be clear that other values also lie within the scope of the invention.
Preferably, the transition from one wall thickness value to another wall thickness value is gradual. This is of benefit to the stability of the process.
In a preferred embodiment, the ratio between the advancement speed of the preform, which is determined by the tube speed-control means, on the one hand, and the output of the extruder, on the other hand, is to be at a substantially constant first value for a first period and to be at one or more values which differ from the first value for a second period, which is considerably shorter than the first period, which cycle is repeated continuously.
In practice, this means, as seen at a point downstream of the expansion mandrel-the stretched tube in each case has a part of great axial length with a uniform first wall thickness and associated diameter, which part is followed by a considerably shorter axial part of the tube in which the wall thickness differs from the said first wall thickness, in particular is of one or more greater values, as seen in the axial direction of the said shorter part. In particular, there is provision for the wall thickness-as seen in the axial direction-to vary between a plurality of values in the latter axial part, so that annular areas which adjoin one another and have different wall thicknesses can be distinguished in the relevant part of the stretched tube.
The method according to the first aspect of the invention can be implemented by periodically varying the output of the extruder, in which case the advancement speed of the preform which is determined by the tube speed-control means is kept substantially constant. This does require an extruder which can be adjusted within a suitable range in terms of its output.
However, the method according to the first aspect of the invention can also be implemented, as is preferred, by keeping the output of the extruder substantially constant and periodically varying the advancement speed of the preform which is determined by the tube speed-control means.
In a preferred embodiment of the method according to the first aspect of the invention, the stretched tube acquires substantially the same axial stretching over its entire length. To achieve this, in the preferred embodiment of the method it is in some cases sufficient to keep the advancement speed of the stretched tube downstream of the mandrel, which is determined by the drawing device, constant, so that the ratio of the advancement speed of the stretched tube downstream of the mandrel, on the one hand, and of the preform upstream of the mandrel, on the other hand, remains substantially constant.
In another preferred embodiment of the method, the advancement speed of the preform upstream of the mandrel, which is determined by the tube speed-control means, varies, and for this reason it is then necessary for the advancement speed of the stretched tube downstream of the mandrel, which is determined by the drawing device, to be varied periodically in such a manner that the ratio of the advancement speed of the tube downstream of the mandrel, on the one hand, and of the preform upstream of the mandrel, on the other hand, is kept substantially constant.
In a variant of the method according to the first aspect of the invention, there is provision for the tube parts with a greater wall thickness not to have the same level of axial stretching as an intervening tube part with a smaller wall thickness, but rather to have a greater level of axial stretching. For this purpose, in the period during which a part of the preform with a greater wall thickness is being forced over the mandrel, or during a section of this period, the ratio of the advancement speed of the stretched tube which is determined by the drawing device, on the one hand, and the advancement speed of the preform which is determined by the tube speed-control means, on the other hand, is greater than in the period during which a part of the preform with a smaller wall thickness is being forced over the mandrel, in such a manner that a tube part having the greater wall thickness acquires a greater level of axial stretching than a tube part with a smaller wall thickness.
To enable the method according to the first aspect of the invention, and in particular according to the variant described above, to be controlled successfully, it is desirable for the tube to undergo its axial stretching in an accurately defined section and, outside this section, for no further, additional axial stretching to be generated in the tube. To achieve this, an advantageous embodiment of the method according to the first aspect of the invention provides for the stretched tube downstream of the expansion part of the mandrel to be cooled in such a manner that the cooled tube no longer undergoes any axial stretching and the generation of the axial stretching is concentrated in the section between the tube speed-control means arranged in the vicinity of the extruder and the downstream end of the mandrel. Preferably, the axial stretching is realized between two tube speed-control means which are arranged at a distance from one another and are both arranged between the extruder and the mandrel.
It will be clear that at the moment at which a preform part with a thickened wall arrives at the upstream end of the mandrel, a possibly critical change occurs in the hitherto stable condition of the method, in particular if the thickened wall part of the preform projects inwards at that moment and thus has a smaller diameter than the adjoining parts of the preform. It would then be expected that the preform part with the greater wall thickness would, as it were, jam on the mandrel, while the thin and still hot part of the preform located immediately downstream of this part would be additionally stretched in the axial direction, possibly to an unacceptable extent.
To solve this problem, in an embodiment of the method according to the first aspect of the invention which is advantageous in practice, the temperature of the preform is controlled in such a manner that a preform part with a greater wall thickness is on average at a higher temperature, measured at a location immediately upstream of the mandrel, than a preform part of a smaller wall thickness which adjoins this part immediately downstream and is therefore already on the mandrel.
If it is assumed that the temperature conditioning substantially consists in cooling the preform, although it is also known from the prior art to supply (relatively small) amounts of heat to the preform upstream of the mandrel, the above-described temperature condition of the preform can in practice be implemented by causing the cooling means, which form part of the temperature-control means, to operate substantially constantly. This can be explained in the following way. In the section between the extruder die and the mandrel it is in fact possible to distinguish between three partial sections. In the first partial section which immediately adjoins the extruder die, it is possible to produce a preform part with a thickened wall. In the adjoining partial section, the preform is subjected to the action of the temperature-conditioning means, in particular to cooling, and in the adjoining third partial section, there is in fact no significant thermal energy supplied to or removed from the preform.
In a preferred embodiment of the method, a preform part with a thickened wall which is formed in the first partial section will move past the temperature-conditioning means in the second section at the same speed as a preform part with a smaller wall thickness. In relative terms, the thicker preform part will therefore be cooled to a lesser extent and will therefore arrive at the mandrel at a higher average temperature; in particular, the temperature of the core of the said thickened preform part will be higher. Due to the higher temperature, the modulus of elasticity will be lower and the thickened preform part will therefore be easier to deform, in relative terms, a fact which in practice can sufficiently compensate for the wall thickening to avoid the above critical situation.
In another preferred embodiment of the method, the speed of the preform is reduced while a preform part with a thickened wall is being formed. In this case too, the said preform part will form in the first partial section. Due to the reduction in speed, that part of the preform which is situated in the second partial section during this period will be subjected to cooling for a longer time than that part of the preform which has already passed through the cooling and is in the third partial section. When the preform part with a thickened wall is complete, the speed of the preform is increased again and the preform part with a thickened wall will pass through the cooling at the said higher speed and will thus be cooled to a lesser extent. When the thickened preform part then arrives at the mandrel, the said part can be deformed easily, while the thin wall part of the preform which is located immediately downstream thereof is in fact relatively rigid. A combination of the two effects makes it possible to carry out the process successfully in a controllable manner.
It can be seen from the above that, on the basis of the temperature of the preform-within a temperature range which is suitable for obtaining biaxial orientation-and the resultant modulus of elasticity of the plastics material of the preform, it is possible to control the axial stretching of the preform. By causing the preform to be at a higher temperature locally, for example at a thicker part thereof as described above, than other parts of the preform at the time of axial stretching, it is possible to ensure that, given a constant axial stretching force exerted on the preform, the hotter part undergoes greater axial stretching than the cooler parts, even if this hotter part has a greater wall thickness. In a practical embodiment, it is possible for the thinner parts of the preform to be at a temperature of approximately 90xc2x0 C. and for a hotter, optionally thicker, part to be at a temperature in the vicinity of 120xc2x0 C.
Surprisingly, it has proven possible to pass the tube through an external calibration device after it has passed the expansion mandrel. In this case, it can be observed that the thickened tube part, on leaving the mandrel, projects outwards with respect to the adjoining parts of the tube and is then pressed inwards by the external calibration device.
The method according to the first aspect of the invention can be carried out in a continuous process, and in this way it is possible to produce a tube from biaxially oriented thermoplastic material with a tube part with a thickened wall at (regular) axial intervals from one another. By then sawing, cutting or suchlike through the tube at the location of the thickened tube parts, it is possible to produce tube sections with, at one or both ends, an end part with a larger wall thickness than the tube body. Furthermore, the invention provides for the said tube sections then to be subjected to a socket-forming operation, in which case an integral socket is formed from an end part with a thickened wall. In a variantxe2x80x94if both end parts are of thicker designxe2x80x94one end part is deformed into a socket and the other end part is used as a spigot. If appropriate, the said spigot is also deformed further, for example is provided with one or more formations, in such a manner that a positively locking socket joint can be obtained.
In a practical embodiment, the tube section has a tube body of uniform cross section and wall thickness with, at one end, an integral socket and, at the other end, a spigot with a wall thickness which is 3-10% greater than the tube body.
Particularly in those embodiments in which the end part with a thickened wall-prior to the formation of the socket has undergone axial stretching which is greater than or equal to the tube body with a smaller wall thickness, the socket obtained has proven to have considerably better properties and a greater load-bearing capacity than the known sockets on such tubes.
Preferably, after the socket has been formed, the axial stretching of the socket is greater than or equal to the axial stretching of the tube body.
Further advantageous embodiments of the method according to the first aspect of the invention are described in the claims and the description.
A second aspect of the present invention relates to a method for producing a tube from biaxially oriented thermoplastic material, which tube has a tube body and, at one or both ends thereof, an integrally formed socket, in which method a prefabricated tube of biaxially oriented thermoplastic material is subjected to a socket-forming operation.
The second aspect of the invention provides for the prefabricated tube to have an end part with a greater wall thickness than the tube body, the axial stretching of the end part prior to the socket-forming operation being equal to or preferably greater than the axial stretching of the tube body. It will be clear that a tube of this nature can be produced using the method according to the first aspect of the invention.
The shape of the socket may be complicated, for example with circumferential ribs of different diameters which, on the inside of the tube, form circumferential areas of different diameters. It is also possible for the wall thickness of the socket, as seen in the longitudinal direction of the tube, to vary and at suitable, e.g. heavily loaded, locations to be thicker than at other locations.
In one possible embodiment, the end part of the prefabricated tube-as seen from its end face-has a plurality of annular areas which adjoin one another and have a wall thickness which fluctuates from one annular area to the next annular area, in which case in a plurality of annular areas the wall thickness is greater than the wall thickness of the tube body. The wall thickness of the end part may thus be of a plurality of values which differ from the wall thickness of the tube body, depending on the socket-forming operation which is yet to be carried out and the requirements which are imposed on the socket.
In a preferred embodiment, an annular area with a greater wall thickness than the tube body is deformed, during the socket-forming operation, into an outwardly bulging groove wall which delimits an internal groove in the tube, which is adapted to accommodate a sealing ring.
A third aspect of the invention relates to the production of a tube from biaxially oriented thermoplastic material, wherein a tubular preform having a wall thickness is extruded from thermoplastic material using an extruder which is provided with an extruder die having an inner core, the inner core defining a hollow space in the preform, wherein the preform is subjected to a temperature conditioning of the preform, so that a tempered preform is obtained having an orientation temperature which is suitable for the thermoplastic material of said preform, and wherein the tempered preform is forced over a mandrel, which mandrel comprises an expansion part, which brings about expansion in the circumferential direction of the preform forced over said mandrel, in such a manner that said preform is transformed into a biaxially oriented tube with thermoplastic material which is oriented in the axial direction and the circumferential direction of the tube, wherein said biaxially oriented tube is cooled, which method comprises the use of multiple preform speed-control means which act on the preform and are arranged at a distance from one another between the extruder and the expansion part of the mandrel, which preform speed-control means each maintain an associated preform advancement speed of the preform, in such a manner that the preform, in between the said preform speed-control means, is axially stretched, thereby reducing the wall thickness of the preform, which method further comprises the use of a drawing device which acts on the tube downstream of the mandrel, which drawing device sets an adjustable tube advancement speed of the tube downstream of the mandrel.
In this known method, at least part of the desired axial stretching of the tube has already been brought about in the preform, before the preform is moved over the expansion mandrel. Then, as it passes over the mandrel, the desired stretching in the circumferential direction is produced, as well as any remaining part of the axial stretching.
In a known method, for example as described in WO 97/10096, two speed-control means, in the form of generally known drawing devices, are arranged upstream of the mandrel, in which case the speed-control means in the vicinity of the mandrel imparts a higher advancement speed to the preform than the other speed-control means. This leads to axial stretching of the preform with reduction of the wall thickness of the preform. In practice, however, this known method of axial stretching has proven insufficiently controllable, with the result that undesirable variations may arise in the preform. Variations of this nature, for example in the cross-sectional shape of the preform, constitute a drawback when the preform subsequently passes over the mandrel.
The third aspect of the invention provides improved control of the axial stretching described above.
According to the third aspect of the invention, the preform, in the section between the speed-control means, in which the preform is axially stretched, is moved through a calibration opening of a calibration device, which calibration device reduces the external diameter of the preform. As a result, the preform acquires an accurately controllable external diameter before the preform reaches the downstream speed-control means and subsequently passes over the expansion mandrel. Furthermore, a significant level of axial stretching can be produced in this section combined with a high level of stability and controllability of the process.
A fourth aspect of the invention relates to a method for producing a biaxially oriented tube from thermoplastic material, in particular polyolefin plastics material, wherein a tubular preform is extruded from thermoplastic material using an extruder which is provided with an extruder die having an inner core, the inner core defining an axial hollow space in the preform, wherein the preform is subjected to a temperature conditioning of the preform, so that a tempered preform is obtained having an orientation temperature which is suitable for the thermoplastic material of said preform, and wherein the tempered preform is forced over a dimensionally stable mandrel, which mandrel comprises an expansion part having an outer surface which substantially corresponds to the surface of a truncated cone, which mandrel brings about expansion of the tempered preform in the circumferential direction of the tempered preform forced over said mandrel, in such a manner that said preform is transformed into a biaxially oriented tube with thermoplastic material which is oriented in axial direction and in circumferential direction of the tube, wherein said biaxially oriented tube is cooled, the method comprising the use of a preform speed-control means which acts on the preform upstream of the mandrel and of a drawing device which is arranged downstream of the mandrel and acts on the tube.
In this known method, the passage of the preform over the expansion part of the mandrel constitutes a problematical part of the production of the tube. In particular, the preform has exhibited undesirable deformations during this part of the production process.
The fourth aspect of the invention seeks to promote the stability of the preform as it passes over the mandrel.
The invention achieves this objective by providing a method, in which the outer surface of the expansion part of the mandrel is provided, at a plurality of locations around the circumference of the expansion part, with elongate grooves and/or ribs which extend in the axial direction, and a film of liquid preferably being formed between the expansion part of the mandrel and the tube.
In an advantageous embodiment, the expansion part of the mandrel is provided with axial grooves which are formed at regular angular intervals, preferably of between 3xc2x0 and 10xc2x0, in the outer surface of the expansion part, and in which the grooves are preferably at most 5 millimetres deep, particularly preferably between 0.5 and 3 millimetres deep.
A fifth aspect of the invention relates to a method for producing a biaxially oriented tube from thermoplastic material, in particular polyolefin plastics material, wherein a tubular preform is extruded from thermoplastic material using an extruder which is provided with an extruder die having an inner core, the inner core defining an axial hollow space in the preform, wherein the preform is subjected to a temperature conditioning of the preform, so that a tempered preform is obtained having an orientation temperature which is suitable for the thermoplastic material of said preform, and wherein the tempered preform is forced over a dimensionally stable mandrel, which mandrel comprises an expansion part having an outer surface which substantially corresponds to the surface of a truncated cone, which mandrel brings about expansion of the tempered preform in the circumferential direction of the tempered preform forced over said mandrel, in such a manner that said preform is transformed into a biaxially oriented tube with thermoplastic material which is oriented in axial direction and in circumferential direction of the tube, wherein said biaxially oriented tube is cooled, the method comprising the use of a preform speed-control means which acts on the preform upstream of the mandrel and of a drawing device which is arranged downstream of the mandrel and acts on the tube.
As is generally known, to force the preform over the mandrel, a considerable tensile force has to be exerted on the stretched tube downstream of the mandrel. When this tensile force is being exerted, it is fundamentally undesirable for the stretched tube to be damaged or permanently deformed.
The fifth aspect of the invention provides for the possibility of exerting a high tensile force by arranging a plurality of drawing devices which drive the stretched tube at the same speed one behind the other downstream of the mandrel.
Also, according to the fifth aspect of the invention, the tube is internally supported at the location where a drawing device, arranged downstream of the mandrel, acts, preferably with the aid of mechanical support means which, at the location where the drawing device acts, comprise one or more support surfaces which move with the tube and bear against the inside of the tube, which support means are preferably attached to the inner core of the extruder.
Preferably, the support surfaces of the support means are driven in the direction of advancement of the tube.
In a variant, it is permissible for the stretched tube to be deformed by the drawing device, namely, in particular, if that part of the tube on which the said device acts subsequently no longer forms part of the tube which is to be marketed. For this purpose, therefore, it is possible for a drawing device to comprise one or more tube-engagement members which can each be moved to and fro over an axial distance, preferably approximately the length of a tube which is to be marketed, and act on part of the tube, so as to deform the tube, and grip the tube securely at that location, each tube-engagement member being assigned an axial displacement mechanism in order to displace the said member and the tube which is secured therein in the axial direction.
The present application also relates to a further number of aspects, which are described in the following paragraphs.
A method for producing a tube section from thermoplastic material, in which a tube section is extruded using an extruder which is provided with an extruder die having an inner core, which inner core defines an axial hollow space in the tube section, in which the tube section coming out of the extruder die, downstream of the extruder die, is internally cooled by means of an internal cooling member, and is externally cooled by means of an external cooling device, in which the internal cooling member internally cools the tube immediately after the tube section has left the extruder die, in which the internal cooling member has a dimensionally stable outer wall with an axial length which is a multiple of the cross-sectional dimension of the tube section, and in which cooling liquid is pressed between the dimensionally stable outer wall and the tube section, in such a manner that a quick-flowing film of liquid is produced between the tube section and the dimensionally stable outer wall, the liquid flowing in the countercurrent direction, i.e. counter to the direction of extrusion, and the film of liquid preferably being at most 3 millimetres thick.
A method for producing a tube section from thermoplastic material, in which a tube section is extruded using an extruder which is provided with an extruder die having an inner core, which inner core defines an axial hollow space in the tube section, in which the tube section coming out of the extruder die, downstream of the extruder die, is internally cooled by means of an internal cooling device which comprises an internal cooling member situated inside the extruded tube, and is externally cooled by means of an external cooling device, the internal cooling member being designed to produce direct contact between a cooling liquid and the tube section, the internal cooling device comprising deaerating means for deaerating the cooling liquid, by means of which the cooling liquid is deaerated before it is fed to the internal cooling member.
A method for producing a tube section from thermoplastic material, in which a tube section is extruded using an extruder which is provided with an extruder die having an inner core, which inner core defines an axial hollow space in the tube section, in which the tube section coming out of the extruder die, downstream of the extruder die, is internally cooled by means of an internal cooling device which comprises an internal cooling member situated inside the extruded tube, and is externally cooled by means of an external cooling device, the internal cooling member being designed to produce direct contact between a cooling liquid and the tube section, the internal cooling member being designed to produce a helical flow of the cooling liquid along the inner wall of the tube section.
A method for producing a tube section from thermoplastic material, in which a tube section is extruded using an extruder which is provided with an extruder die having an inner core, which inner core defines an axial hollow space in the tube section, in which the tube section coming out of the extruder die, downstream of the extruder die, is internally cooled by means of a cooling liquid which is brought into direct contact with the tube section, and is externally cooled by means of an external cooling device, a cooling liquid with a low surface tension being used, the cooling liquid preferably being water to which one or more additives which reduce the surface tension have been added.
A method for producing a tube section from a polyolefin plastics material, in which a tube section is extruded using an extruder which is provided with an extruder die having an inner core, which inner core defines an axial hollow space in the tube section, in which the tube section coming out of the extruder die, downstream of the extruder die, is internally cooled by means of an internal cooling device which comprises an internal cooling member attached to the inner core, and is externally cooled by means of an external cooling device, a heating medium being present in the hollow space in the tube section downstream of the internal cooling member, for the purpose of increasing the temperature of the layer on the inside of the tube section which has been cooled by the internal cooling member, the heating medium preferably being a liquid, if appropriate with an added substance which reduces the surface tension, at a temperature of between 90 and 100xc2x0 C.
A method for producing a tube section having a wall layer made from crystalline thermoplastic material, in which a tube section is extruded using an extruder which is provided with an extruder die having an inner core, which inner core defines an axial hollow space in the tube section, in which the tube section coming out of the extruder die, downstream of the extruder die, is internally cooled by means of an internal cooling device which comprises an internal cooling member situated in the tube, and is externally cooled by means of an external cooling device, a multilayer tube being extruded with at least one wall layer of amorphous thermoplastic material on the inside of the wall layer consisting of crystalline thermoplastic material, the crystalline wall layer being made, for example, from polyethylene, and the amorphous wall layer being made, for example, from polyvinyl chloride.
A method for producing a biaxially oriented tube from thermoplastic material, in particular polyolefin plastics material, comprising the extrusion of a preform from thermoplastic material using an extruder which is provided with an extruder die having an inner core, the inner core defining an axial hollow space in the preform, and then forcing the preform over a mandrel, which mandrel comprises an expansion part which brings about expansion of the tube in the circumferential direction, the extruder die being provided with means for controlling the wall thickness of the preform coming out of the extruder die, and an ultrasonic device for measuring the wall thickness, which is arranged along the outside of the tube, being provided between the extruder die and the mandrel, for the purpose of measuring the wall thickness and shape of the cross section of the extruded preform, a layer of cold liquid being produced on the inside of the preform at the location of the wall-thickness measuring device, the temperature of the layer of cold liquid preferably being at most 50xc2x0 C.
A method for producing a biaxially oriented tube from thermoplastic material, in particular polyolefin plastics material, comprising the extrusion of a preform from thermoplastic material using an extruder which is provided with an extruder die having an inner core, the inner core defining an axial hollow space in the preform, and then forcing the preform over a dimensionally stable mandrel in the axial direction, which mandrel comprises an expansion part which brings about expansion of the preform in the circumferential direction, the preform being forced over the mandrel by means of a speed-control means which engages on the preform upstream of the mandrel and by means of a drawing device which is arranged downstream of the mandrel, the preform being heated in a manner which can be controlled by circumferential sector upstream of the mandrel, this controllable heating per circumferential sector being effected by means of microwave radiation.
A method for producing a biaxially oriented tube from thermoplastic material, in particular polyolefin plastics material, comprising the extrusion of a preform from thermoplastic material using an extruder which is provided with an extruder die having an inner core, the inner core defining an axial hollow space in the preform, and then forcing the preform over a mandrel in the axial direction, which mandrel comprises an expansion part which brings about expansion of the preform in the circumferential direction, and a run-off part downstream of the expansion part, which run-off part is substantially constant in cross section, the preform being forced over the mandrel by means of a speed-control means which engages on the preform upstream of the mandrel and by means of a drawing device which is arranged downstream of the mandrel, and the run-off part having an axial length which is a multiple of the wall thickness of the oriented tube.
A method for producing a biaxially oriented tube having a wall layer made from polyolefin plastics material, comprising the extrusion of a preform from thermoplastic material using an extruder which is provided with an extruder die having an inner core, the inner core defining an axial hollow space in the preform, and then forcing the preform over a mandrel in the axial direction, which mandrel comprises an expansion part which brings about expansion of the preform in the circumferential direction, and a run-off part downstream of the expansion part, which run-off part is substantially constant in cross section, the preform being forced over the mandrel by means of a speed-control means which engages on the preform upstream of the mandrel and by means of a drawing device which is arranged downstream of the mandrel, a multilayer preform being extruded, which incorporates a plurality of wall layers with different properties, at least one of which is made from polyolefin plastics material, at least one of the wall layers being subjected, for example, to a crosslinking treatment, preferably an inner and/or outer wall layer, which preferably contains additives which promote crosslinking.
A method for producing a biaxially oriented tube from polyolefin plastics material, comprising the extrusion of a preform from thermoplastic material using an extruder which is provided with an extruder die having an inner core, the inner core defining an axial hollow space in the preform, and then forcing the preform over a mandrel in the axial direction, which mandrel comprises an expansion part which brings about expansion of the tube in the circumferential direction, and a run-off part downstream of the expansion part, which run-off part is substantially constant in cross section, the preform being forced over the mandrel by means of a speed-control means which engages on the preform upstream of the mandrel and by means of a drawing device which is arranged downstream of the mandrel, the tube being subjected to a crosslinking treatment downstream of the expansion part of the mandrel, preferably only a wall layer adjoining the inside and/or outside of the tube being subjected to a crosslinking treatment.
A connection of two tubes of biaxially oriented thermoplastic material, in particular polyolefin plastics material, in which the tubes, at their ends which face towards one another, are each provided with an integrally formed socket which preferably has a larger internal diameter than the adjoining part of the tube, and in which a connecting-tube body is provided, having two axial ends which each fit into a socket of a tube which is to be connected, and in which the socket of each tube is heat-shrunk onto that end of the connecting-tube body which fits into said socket securely.
The connection according to the previous paragraph, in which the connecting-tube body is provided, at each of its ends, with heater means for heating the socket which has been pushed over it, for example one or more electrical heater elements, for example heater wires, and/or one or more elements which can be heated from the outside, for example metal elements which can be heated via induction.
The connection according to one or more of the previous paragraphs, in which the heater means lie at a distance from the free end of the connecting-tube body.
The connection according to one or more of the preceding paragraphs, in which the outer surface of each end of the connecting-tube body is profiled in order to create a dimensionally stable connecting component between the connecting-tube body and the socket of the tube.
The connection according to one or more of the preceding paragraphs, in which the connecting-tube body consists substantially of plastics material.
The connection according to one or more of the preceding paragraphs, in which the internal diameter of the connecting-tube body is substantially equal to the internal diameter of that part of each tube which lies outside the socket.
A tube of biaxially oriented thermoplastic material, which plastics material has a stretch ratio, in the axial direction and in the circumferential direction, with respect to the preform from which the tube is made, the sum of the stretch ratio in the axial direction and in the circumferential direction being between 4 and 6, preferably between 4.5 and 5.5, particularly preferably being approximately 5.
A tube of biaxially oriented thermoplastic material, for example polyethylene (PE), which plastics material has a stretch ratio, in the axial direction and in the circumferential direction, with respect to the preform from which the tube is produced, the sum of the stretch ratio in the axial direction and in the circumferential direction being between 4 and 6, preferably between 4.5 and 5.5, particularly preferably being approximately 5, and the stretch ratio in the axial direction being in a relationship of 3:2 with respect to the stretch ratio in the circumferential direction.
The abovementioned measures and other measures provided according to the invention are described in the following description and will be explained below, in particular with reference to the drawings.